The present invention relates to an optical tape which is capable of recording and reproducing information by the irradiation of light. More particularly, the present invention relates to an optical tape having notably improved recording and reproducing characteristics for information to be recorded.
Hitherto, magnetic recording media have been commonly used as information-recording tape. Recently, however, a request for improvement of information-recording density has been enlarged, and as an answer to such request, several types of optical tapes utilizing optical recording/reproducing systems have been proposed.
For example, Japanese Patent Application Laid-Open (Kokai) No. 57-33447(1982) discloses a tape obtained by coating a plastic base film with a Te--As--Se metal film so as to have about 1,000 .ANG. in thickness and selectively forming the micropores in the metallic film by laser beams, and a tape obtained by coating the plastic base film with a mixture of carbon and nitrocellulose.
Japanese Patent Application Laid-Open (Kokai) No. 62-132251(1987) discloses an optical tape produced by depositing Te--Se alloys, Se--In--Sb alloys or Ag--Zn alloys on a polyimide base film by means of Re-magnetron sputtering, and further forming thereon a protective layer.
Also, in Japanese Patent Application Laid-Open (Kokai) No. 1-286130(1989) is disclosed an optical tape in which a magnetic layer is formed on one side of a base film and an anti-reflective layer is formed on the opposite side of the base film.
Various kinds of optical tapes such as mentioned above, have been proposed, but they involved the technical problems concerning the substrates. Each of the substrate used for the optical tapes had its own merits and demerits, and a substrate which is favorably put to practical use has not been available. For instance, it is preferable that the surface of the recording layer is as smooth as possible, but if the film surface is smooth, film-to-film slipperiness deteriorates, thereby encumbering commercialization of the optical tape.
Polyester film is excellent in chemical, mechanical and thermal properties, and also it is possible to obtain relatively easily a polyester film with a flat and smooth surface, so that polyester film is suited for use as a substrate of an optical tape. However, in the course of studies on manufacture of an optical tape by using such polyester film, it was found that the polyester film was incapable of putting to practical use to manufacture an optical tape because of poor slipperiness that is encountered in the polyester film-producing step and optical recording layer-forming step and in the optical tape-running system. Further, the optical tape obtained by using a smooth polyester film, due to poor slipperiness, would suffer many scratches causative of drop out on the film surface on the read-out side or on the recording layer surface, resulting in poor performance of the optical tape.
Some of the present inventors had proposed an optical tape capable of optical recording and reproducing information, the optical tape comprising a base film and a recording layer disposed on the base film, a center line average roughness on the surface of the base film, in which the recording layer is formed, being not more than 50 .ANG. (Japanese Patent Application Laid-Open (Kokai) No. 4-163736).
Some of the present inventors had also proposed an optical tape capable of optical recording and reproducing information, the optical tape comprising a base film and a recording layer disposed on the base film through an intermediate layer, the center line average roughness on the surface of the intermediate layer, in which the recording layer is formed, being not more than 50 .ANG. and the center line average roughness on the surface of the base film, which is opposite to the intermediate layer being 50 to 1,000 .ANG. (Japanese Patent Application Laid-Open (kokai) No. 4-163737).
An optical tape having excellent durability and heat resistance in addition to excellent recording and reproducing performance has been earnestly desired.
As the results of the strenuous studies, it has been found that by using a substrate whose center line average roughness (Ra.sup.A) on one side thereof is 0.005 to 0.5 .mu.m and tensile strength (F.sup.5) in the longitudinal direction (machine direction) is not less than 8 kg/mm.sup.2, the obtained optical tape has excellent recording and reproducing performance as well as excellent durability and heat resistance. The present invention has been attained on the basis of this finding.